First Evidence of Resistance to ALS ‐Inhibiting Herbicides in Hirschfeldia incana : Insights From Whole‐Plant Responses and Structural Modelling

Abstract

Hirschfeldia incana is a widely distributed Brassicaceae species that has become a weed in temperate cropping systems worldwide. In the Argentine Pampas, its increasing prevalence in winter cereals has raised concerns because acetolactate synthase (ALS)‐inhibiting herbicides are extensively used for broadleaf weed control. This study aimed to characterise the mechanism of resistance to ALS‐inhibiting herbicides in two putatively metsulfuron‐methyl‐resistant H. incana populations and evaluate their cross‐resistance patterns. This work constitutes the first report of ALS‐inhibitor resistance in H. incana and provides the first ALS coding sequences available for this species. Sequencing identified the Trp‐574‐Leu substitution as the only non‐synonymous mutation associated with resistance, while five synonymous polymorphisms distinguished the resistant populations, indicating two distinct resistant haplotypes. Whole‐plant dose–response assays demonstrated broad cross‐resistance to nine herbicides representing the five chemical families of ALS inhibitors, with resistance indices ranging from 10 to 22 and exceeding 31.6 for flucarbazone‐sodium. Cross‐resistance extended to the sulfonylaminocarbonyltriazolinone and pyrimidinyl benzoate families despite the absence of previous field exposure to either group. Molecular docking predicted reduced binding affinity of all evaluated herbicides towards the Trp‐574‐Leu ALS variant, although the magnitude of this effect varied among compounds and was accompanied by herbicide‐specific changes in the ligand‐binding environment, including loss and compensatory reorganisation of noncovalent interactions. These findings demonstrate that selection imposed by a limited number of ALS‐inhibiting herbicide families can compromise the efficacy of chemically distinct ALS inhibitors, highlighting the need for integrated weed management programs based on diversification of herbicide sites of action across crop rotations.